JPH0125823B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a method for heat treating metals.

One of the heat treatment methods commonly used commercially is carbonitriding of low carbon steel. If the part to be carbonitrided has a relatively thin cross-section, the part is prone to physical distortion. Attempts have therefore been made to find alternative heat treatment methods for carbonitriding to avoid this distortion problem. One such method is ferrite carbonitriding. If mild steel parts are treated with ferrite carbonitriding, the problem of distortion can be overcome.
It has also been found that a scuffing resistant surface finish can be obtained. One drawback of the ferrite carbonitriding process is that parts treated by this process tend to have poor indentation resistance. Another method to overcome the above-mentioned distortion problem is the austenitic carbonitriding process. Furthermore, by applying austenite carbonitriding treatment to the mild steel member, good scuffing properties and indentation resistance can be obtained.

To austenitic carbonitriding components made of mild steel or other low carbon steels, ammonia has traditionally been added to the atmosphere generated by an endothermic gas generator to create the required atmosphere, and The usual practice was to feed the resulting gas mixture (or two separate gas streams) into the furnace (usually a closed quenching furnace) in which the treatment took place. However, endothermic gas generators have certain drawbacks. Indeed, endothermic gas generators can be expensive to maintain, and furthermore, skilled operation is often required for such purposes. Endothermic gas generators are relatively expensive items of capital equipment with a fixed lifespan, and are very bulky and require floor space that could otherwise be used for other purposes as a heat treatment plant. It blocks the It is an object of the present invention to provide a method for providing an atmosphere suitable for austenitic carbonitriding of steel or other ferrous metals without the use of endothermic gas generators.

The method of austenitic carbonitriding of ferrous metal in a heat treatment furnace of the present invention involves adding nitrogen, ammonia, and liquid or gaseous methanol to the heat treatment furnace in a ratio of 2 moles of nitrogen to 2 moles of methanol and 1 mole of ammonia. By introducing 7-11 vol% carbon monoxide, 30-40 vol% hydrogen, 6-10 vol% free ammonia, and 1-2 vol% carbon dioxide suitable for austenite carbonitriding. carbon and 2
Creating an atmosphere containing ~3% by volume methane, the balance nitrogen and water vapor, and having a dew point in the range -5°C to +5°C, and maintaining this atmosphere at a temperature in the range 690° to 750°C It is characterized by The methanol may be dropped into the furnace,
Alternatively, it may be vaporized before entering the furnace, i.e. upstream, and introduced into the furnace as a vapor.

Generally, the nitrogen used in the process is stored in a thermally insulated container in liquid form and vaporized upstream of the furnace.

In the furnace the methanol decomposes and a gas mixture consisting of nitrogen, hydrogen, carbon monoxide, carbon dioxide, water vapor and methane is formed. Additionally, most, if not all, of the ammonia decomposes to form nitrogen and hydrogen. The atmosphere of the furnace then contains nitrogen, hydrogen, carbon monoxide, carbon dioxide, water vapor, methane and ammonia in the proportions indicated in volume %. Such an atmosphere may be, for example, 120 cubic feet (3.4 liters) of nitrogen per hour in a closed quenching furnace at 700°C.
It can be formed by introducing 60 cubic feet (1.7 liters) of ammonia and 2 liters of methanol (measured as a liquid) for one hour. As an example, the inventors may combine 36% by volume hydrogen, 9% by volume carbon monoxide, 8.5% by volume ammonia, 2.6% by volume methane, 1.3% by volume carbon dioxide, and the balance nitrogen. , and it was confirmed that a furnace atmosphere having a dew point in the range of -2°C to +2°C was generated. If desired, the proportion of free ammonia in the furnace can be controlled using a dissociating pellet.

Generally, the part to be carbonitrided may be held in the atmosphere at a temperature between 690°C and 750°C for a period of 2 hours. During this time a relatively thin white outer surface layer forms on the component (or other workpiece) being processed. This outer layer is of the type known by those skilled in the art as epsilon compounds and contains oxygen, nitrogen and carbon. The outer surface layer has scuffing resistance. In addition to the outer surface layer, there is an internal carbonitrided case surrounding the ferrite core. After quenching the part with oil, the case has two zones, one austenite and the other martenside. Therefore, in order to optimize the mechanical properties of the case, it is desirable that the austenite be transformed into low bainite or marteinsite. The transformation to low bainite is generally carried out isothermally by tempering at a temperature of 250°C or higher for at least one hour (preferably at a temperature of 300°C or higher for at least two hours). be exposed. The transformation to martensite is accomplished by lowering the temperature of the component to -70 DEG C. or below, and then allowing the temperature of the component to return to ambient temperature. In order to prevent the austenite from stabilizing, it is desirable to start lowering the temperature of the member to −70° C. or lower within 2 hours after the quenching (by oil) is completed.

Generally, if the component is made of mild steel and if the component is held in the furnace atmosphere at a temperature of 700° C. for two hours, the (outer) layer of epsilon compound will be 0.0010 to 0.0012 inches (0.0254 to 0.0254 and the (inner) case has a depth of 0.005 to 0.006 inch (0.127 to 0.152 mm).

A representative microhardness distribution curve of a mild steel sample subjected to austenitic carbonitriding according to the present invention is shown in the accompanying drawings. This figure is a graph showing how the hardness (HV x 0.1) of the sample changes as the depth of the sample increases.

Results for three samples are shown in the graph above. All samples were held for two hours in a 700°C atmosphere of the type described hereinabove. The austenite of the first sample was isothermally transformed to low bainite by tempering at 300°C for 2 hours, and then the austenite of the second sample was transformed into a second sample. By lowering the temperature of the sample to -70℃, that is, by performing sub-zero treatment at -70℃, it is transformed into martensite.
On the other hand, the third sample was not subjected to any treatment to transform its austenite. The first sample has a more ductile surface but less hardness compared to the second sample.

The present invention also includes within its scope ferrous metals treated by the inventive austenitic carbonitriding process described hereinabove.

[Brief explanation of drawings]

The drawing is a graph showing a typical microhardness distribution curve of a mild steel sample subjected to austenite carbonitriding according to the present invention.

Claims (1)

[Claims] 1. By introducing nitrogen, ammonia, and liquid or gaseous methanol into a heat treatment furnace at a ratio of 2 moles of methanol and 1 mole of ammonia to 2 moles of nitrogen, 7 to 11% by volume suitable for austenite carbonitriding is produced. carbon monoxide, 30-40% hydrogen by volume, 6-10% free ammonia, 1-2% by volume
Contains vol% carbon dioxide, 2 to 3 vol% methane, and the balance nitrogen and water vapor, and has a temperature of -5°C to +
creating an atmosphere having a dew point in the range of 5°C;
A method of austenitic carbonitriding of ferrous metals in a heat treatment furnace in which this atmosphere is maintained at a temperature ranging from 690° to 750°C.